The invention relates to devices for gas-tight and fluid-tight separation of different spaces, in particular piston pressure devices.
To generate high pressure for mechanical work operations or for experimental purposes, piston arrangements are commonly used in which, through mechanical forward thrust of a piston in a hydraulic fluid, pressure is created and transmitted to the control element that is to be actuated by suitable means (eg high-pressure line). For effective build-up of pressure and to maintain pressure stability, the piston must be sealed off from the wall of the piston vessel (cylinder) for the hydraulic fluid. Here it is necessary that the sealing, even through the effect of the working pressure, should not let any hydraulic fluid out of the piston compartment into the outer compartment or into the sealing. Hydraulic fluids of high viscosity, eg oil and glycerin, are in widespread use for generating high pressure. The high viscosity of these hydraulic fluids makes it possible to produce effective sealings from elastomers with relatively little effort.
The use of high-viscosity pressure media in high-pressure devices simplifies the sealing of the system, but the disadvantage is that filling and venting the system is difficult and time-consuming. One problem is the avoidance of gas pockets in the high-pressure system, especially in engineering process applications with high demands for stability.
Physical engineering or chemical engineering processes may require pressure fluctuations to be less than {fraction (1/100)} at.
Low-viscosity hydraulic fluids like ethanol and petroleum ether are known as suitable, clean and non-toxic pressure fluids. The disadvantage of low-viscosity hydraulic fluids, however, is that they generally have a low boiling point, so it is possible for vapor to form in the piston region. Furthermore, it is more difficult, especially in high-pressure applications, to produce effective sealing with low-viscosity hydraulic fluids.
In the case of high-pressure devices for work apparatus (eg hydraulic excavators) with which especially high regulating forces have to be produced, the operation of hydraulic pistons with armored sealing is familiar. Sealing of this kind is often produced as a combination of elastic and plastic elements. For example, it consists of a plastic that, through contact with the wall and at sufficiently high pressure, ensures sealing, and of a helical spring that is partly enclosed by this plastic and produces sufficiently high contact pressure for sealing at low pressure. At high pressure the direct effect of the pressure medium on the plastic sealing must produce the sufficiently high contact pressure. Sealing armored with spring elements exhibits decisive drawbacks. In the open helical spring region it is easy for air occlusions to form, which can lead to disturbances in the build-up of pressure when filling the pressure system (hydraulic system) and in later operation. Venting the sealing is only possible within time limits and involves considerable effort. Furthermore, spring armored sealings are restricted to certain minimum sizes (no possibility of miniaturization). Corrosive effects can appear on the armoring. Finally there is the possibility of the plastic being damaged by the spring armoring at high pressure (xe2x89xa7100 at), or of irreversible plastic deformation of the spring at very high pressure.
Conventional sealing devices also exhibit the following disadvantages. In the case of fluid sealings, leaks for gases will appear. This is problematic for the event that a gas is released in the hydraulic fluid or the hydraulic fluid is of low viscosity. With conventional pressure devices this means that gases exit into the sealing or even into the outer compartment. A further problem is that with conventional pressure devices there may be a fluid film to reduce friction between the sealing and the cylinder wall. This fluid film means in turn a change in the amount of hydraulic fluid and thus inaccuracy and instability in pressure generation. Numerous pressure devices are also intended for operation with standard hydraulic fluids. Consequently use of such sealings is not possible in compression measurements on random fluid samples with, possibly, unfavorable chemical properties for the sealings. Further disadvantages of conventional pressure devices are their lack of miniaturization capability (structure too complex) and the formation of gas occlusions in the packing. Contamination of the sealing by gases or fluids from the pressure medium that is used also leads to impurities in subsequent operation with other pressure media.
Sealings are also known that consist of an elastically and a plastically deformable component (see DE-OS 33 15 050 for example). Here, at low pressure, the plastically deformable component is pressed against the cylinder wall through the effect of the elastically deformable component. The plastically deformable component is also subjected to the pressure of the directly contacting pressure medium, whereby at higher pressure sufficiently high contact pressure is to be ensured against the cylinder wall that is to be sealed. Seeing as the plastically deformable component is in direct contact with the fluid over a large surface and the edges of the plastically deformable component against the cylinder wall or the piston are exposed to the pressure medium, this can penetrate into the packing. That makes this kind of sealing unsuitable for accurate and reproducible pressure settings in the high-pressure region and for simple venting of the packing.
Piston cylinder configurations with pretensioned sealing elements are known from DE-OS-1 901 247, DE-OS-40 08 901 and DE-OS-33 15 050.
The object of the invention is to propose an improved piston pressure device that is fluid-tight and gas-tight, that allows creation of highly stable pressure in the sealed space and whose effectiveness is independent of the viscosity of adjacent fluids.
This purpose is solved by a piston pressure device with the features of patent claim 1. Advantageous embodiments of the invention are defined in the dependent claims.
The invention is based on the idea of a piston pressure device in which a piston is anchored so as to be axially movable on a piston rod, whereby a recess for form-closed holding of a sealing device is created between the facing ends of the piston and piston rod. In the radial direction the recess is covered by a cylinder wall. The sealing device, primarily an elastic component of the sealing device, is largely protected by the piston against direct contact with the pressure medium. Nevertheless, the compressive forces of the pressure medium on the face end of the piston can be transmitted to the sealing device because of the axial motion of the piston referred to the piston rod. The result is compression of the sealing device in the recess, pressing it against the cylinder wall.
The outer diameter of the piston is matched to the inner diameter of the cylinder in such a way that axial mobility of the piston in the cylinder is ensured in all operating states of the piston pressure device. The formation of a fit between the piston and the cylinder means that the pressure exercised by the pressure medium on the piston is essentially created on the face end of the piston. Although pressure medium is able to penetrate between the piston and the cylinder wall, it is of a small and reproducible amount. Further penetration of the pressure medium into the sealing device is virtually excluded, however, compared to conventional piston pressure devices, because the sealing device is not immediately adjacent to the pressure medium (with the exception of the residual, negligible gap through the fit between the piston and the cylinder).
The sealing device is preferably a pressure-activated sealing device containing an elastomer and a material that is plastically deformable under pressure. The elastomer is at least in part enclosed by the deformable material. The elastomer is preferably formed of a synthetic rubber and the material deforming under pressure of a plastomer, preferably polytetrafluorethylene (PTFE), or tetrafluorethylene copolymers. The components of the sealing device interact as follows to form the pressure-activated packing.
When there is a pressure difference, equal to or somewhat greater than zero, between two adjacent spaces separated by the sealing device (eg inner and outer compartments of a piston pressure device), the elastomer produces sealing. For this purpose the elastomer is selected with a certain Shore hardness and pretensioned so that sufficiently high elastic pressure is exerted on an adjacent wall (eg piston cylinder wall) to produce the required tightness. In this state the shape elasticity of the elastomer is utilized for sealing, while the material deformable under pressure does not contribute to creating the elastic pressure against the wall. Differing from the properties of conventional spring armorings, the elastomer has a positive compressibility characteristic, ie it is easily deformable and elastic at slight pressure and non-deformable at high pressure.
As soon as the pressure difference between the spaces increases (eg through mechanical actuation of a piston device fitted with the sealing device), the sealing device is compressed. Since the compressibility of the first component (the elastomer) is slight, the increasing pressure difference, with suitable transmission of the compressive forces to the sealing device, produces deformation of the second component (socalled Bridgeman effect). This deformation leads to gas-tight and fluid-tight contact between the limiting wall and the deformable component, and in particular to occlusion of all and any unevenness on the wall. The deformable material possesses as low as possible a coefficient of friction to create a wiping dry seal. This requirement is also satisfied by PTFE or tetrafluorethylene copolymers.
The sealing device is preferably of a shape that, depending on pressure, ensures an optimal seal by one of the components. For this purpose the sealing device is formed of a solid of the material deformable under pressure in which the elastomer is embedded at least in part. The elastomer is arranged in the sealing device so that there is no direct contact with a vessel wall that is to be sealed. Instead the elastomer is separated from the surface of the wall by the material deformable under pressure. In use of the invention in a piston pressure device, the body of the sealing device preferably takes the form of a cylindrical sleeve of the material deformable under pressure, in which an elastomer ring is axial-symmetrically embedded.
The piston pressure device according to the invention contains in a piston casing a piston shaft with a piston that is linked to the piston shaft so as to be axially mobile by a means of anchoring. Arranged shell-like in the recess between the piston and the piston shaft is the sealing device, which is pretensioned when the piston starts to operate and is pressed against the wall of the piston casing as pressure builds up.